The mobile communication networks are constantly under development by adding new features and improving already existing ones. The work is bound by the standardization between the manufactures of such networks for allowing the manufacturing of user equipment that works in mobile communication networks from different manufacturers. Thus, new features must be developed in accordance with already agreed features.
Coordinated multipoint (CoMP) transmission/reception is one of the features that are currently being specified to be a part of the Long Term Evolution (LTE) standard defined by 3rd Generation Partnership Project (3GPP). In multi-cell network technologies supporting frequency reuse, the network performance is interference limited, especially at the cell edge. An example of such technology is the LTE. The coordinated multipoint operation aims at strengthening the received signal and mitigating inter-cell interference. In the CoMP users may be served by a plurality of transmission points (TP) at the same time. The information transmitted from the plurality of transmission points can be the same or different. According, to the 3GPP technical report on CoMP, TR36.819, a point is a set of geographically co-located transmit antennas and the sectors of the same site may correspond to different points. It should be noted that a cell, may be formed by one or multiple points. In the field of CoMP the features relating to joint transmission, dynamic point selection and coordinated scheduling and beam forming are currently studied.
In joint transmission (JT) CoMP two or more points transmit simultaneously to a CoMP user. Both coherent and non-coherent JT-CoMP have been studied. Coherent JT-CoMP aims at coherent combining of received signal components from multiple transmission points. In order to achieve coherent combining gains, inter-point phase combiner user equipment feedback is required in addition to per transmission point channel state information (CSI) feedback. In non-coherent JT-CoMP, a user equipment (UE) receives signals from multiple transmission points. UE feeds back CSI on a per transmission point basis. Since non-coherent JT-CoMP does not aim at coherent combining gains, it does not require inter-point phase combiner feedback from the UE.
Dynamic point selection (DPS) refers to a scheme wherein the transmission point is switched according to changes in signal strength. In coordinated beamforming/scheduling (CB/CS) the scheduling decisions of neighbor points are coordinated in order to reduce interference. In principle, all schemes may include blanking/muting, which means that one or more transmission points are blanked/muted to decrease the interference.
It is desired to determine intra- and inter-cell DL CoMP schemes operating in homogeneous and heterogeneous configurations. Four main scenarios have been studied so far: intra-site (scenario 1), inter-site with high power remote radio heads (RRH) (scenario 2), low power RRH within the coverage of the macro cell, without and with the same cell ID (scenarios 3 and 4, respectively). The work addresses both frequency division duplex (FDD) and time division duplex (TDD). Hence unified solutions should be targeted, as it is typical, for example, in the LTE specifications. Hie network deployments thus include of a plurality of transmission points, and each transmission point has its own antenna configuration, such as e.g. in terms of the number of antennas and the type of antennas, for instance cross-polarized (XP) antennas or uniform linear arrays (ULA), with either close (e.g. λ/2) or large (e.g. 4λ) separation between elements. Deployments with lower power nodes (LPN) or remote radio heads typically assume that the corresponding transmission points are geographically non-co-located.
It has been agreed that per point channel quality indicator (CQI) feedback is fed back per each transmission point in a CoMP measurement set of size N and additional feedback of aggregated joint CQIJT is currently under discussion.
A CQI depends on the transmission hypotheses made by the UE at a given time and the interference assumptions used for deriving per point CQIs. However, the CQIs reflect always the channel gains. Thus, per point CQIs reflect the individual link gains whereas the aggregated CQI reflects the sum of the channel gains and the way the signals add up at the receiver. For example, with the hypothesis of muted interfering points within the measurement set, the per cell channel quality indicator fed back for the n-th transmission point is defined as:
      CQI    n    =            S      n                      σ        2            +      I      
where Sn denotes signal power received from point n, σ2 denotes noise variance at the users and I denotes interference power received from out of the CoMP measurement set of transmission points. With the knowledge of these per point CQIs, as an implementation option, the network may derive a CQI estimate e.g. as a sum:CQIest=Σn=1NCQIn 
This, however, does not include knowledge on the constructive/destructive addition of the signals from different transmission points. Therefore, the process of feeding back the additional aggregated CQIJT would result in improved system performance since the latter CQI precisely captures the constructive/destructive addition of received signal components the network is not aware of.
The single stream (i.e. rank-1) JT-CoMP transmission equation for N transmission points and single sub-carrier can be expressed as:y=Σn=1NcnHnwnx+η
where x is a transmitted symbol; Hn is a channel between n-th transmission point and receiver; wn is a tall preceding vector and cn is the combiner applied at the n-th transmission point; y is a received signal; η is a noise term. Note that c1=1 to avoid ambiguity and without any loss of generality.
As mentioned, the CQI is a function of the channel gains. As an example, an effective channel gain for a two point JT-CoMP single stream (i.e. rank-1) transmission reads:GJT-CoMP=|heff1|2+|heff2|2+2Re(heff1Hc2heff2)
wherein the first term is the channel gain from point 1, the second term is the channel gain from point 2 and the third term is constructive/destructive addition of channels from points 1 and 2.
In the equation heffn=Hnwn is the channel after preceding from transmission point n and c2 is the possible combiner weight between the two points. The last term describes the constructive/destructive addition of the channels from the two points. If the term is negative the addition is destructive and when the term is positive the addition is constructive. The constructiveness depends on the phase between the effective channel vectors and makes the addition positive/negative with 50% probability assuming no inter-point feedback information is used. Per-point CQIs capture only the individual channel gains, that is, in the above example with two points, the first two terms in Equation (1). Thus per-point CQIs do not capture the constructive/destructive addition and the last term of the channel gains in Equation (1) results in CQI mismatch.
The distribution of ratio CQIJT/CQIest is illustrated in FIG. 1. It can be seen that a negative combiner can result in the loss of as much as 10 dB, whereas positive combiner CQIC can bring at the most 3 dB gain. Therefore, it is beneficial to be in the “constructive combing area”, where in addition to positive combining gain, the CQIest offset is on average as small as +−1.5 dB. The question is however, how to guarantee the transmission to be in “constructive combing area” with minimal UE feedback.
For both per-point CQIs and for joint CQI changes in the last term in equation (1) during CQI reporting interval causes CQI mismatch. There is typically an outer loop link adaptation (OLLA) algorithm that, follows through ACK/NACK reporting the signal condition changes resulting from channel gain variations, interference condition variations and in case of JT transmission also changes in the channel combining gain. If constructive/destructive channel additions vary in time and frequency from one extreme to another extreme it makes the task of OLLA more difficult and may result overall in poor system throughput performance. However, OLLA is a powerful tool to correct CQI error if the error direction does not fluctuate too much. Thus, if the addition of the signals can be made always, or most of the time, positive this would result in a positive effect for both aggregated CQI feedback assumption and for per-point CQI feedback assumptions. For aggregated CQI, the positive effect comes from signal-to-noise and interference ratio (SINR) and CQI level increase. For per-point CQIs, the receiver SINR is increased but not the per-point CQI reports themselves. However, as the CQI error direction is always positive, OLLA is able to compensate and the performance increase results from that.
The above mentioned OLLA algorithm is typically used for single point transmissions and is described in the following for the sake of the clarity. OLLA algorithm as such for single point transmissions is known to a person skilled in the art.
One type of OLLA algorithm targets at controlling the block error rate (BLER) target for the first transmission to the user equipment. More advanced OLLA algorithms aim at controlling the residual BLER target at the first re-transmission so the effect of the HARQ gain is captured by the algorithm. The OLLA algorithm adjusts an offset factor denoted by A according to the following rules:                In case a positive acknowledgment (ACK) is received for a first transmission, then decrease A by a quantity AstepDown in decibels.        In case a negative acknowledgement (NACK) is received for a first transmission, then increase A by a quantity AstepUp in decibels.        
Only ACK/NACK's from first transmissions are used here for adjusting the offset A. The ratio between the parameters AstepUp and AstepDown determines the BLER target that, the algorithm will converge to:
  BLER  =      1                  AstepUp        AstepDown            +      1      
For certain desired BEER target (e.g. 10%) and a known AstepUp, one may compute AstepDown as:
  AstepDown  =            AstepUP      ·      BLER              1      -      BLER      
The OLLA algorithm operates on top of the inner loop link (i.e. CQI) adaptation for a given UE by subtracting the offset value A from the reported CQI:CQIapplied−CQIreported−A [dB]
It is further ensured that the OLLA offset factor A remains within a given predetermined range [dBmaxOuterLoopLAOffset, dBmmOuterLoopLAOffset] (e.g. [−1 dB, 3 dB]) to avoid possible divergence of the algorithm. Since the OLLA algorithm is recursive, parameters need to be initialized, e.g. AstepUp=0.5 dB. Optimum tuning of OLLA parameters can be made through system level simulations.
A drawback of the prior art is that without combiner phase indicator feedback information constructive combination cannot be guaranteed and OLLA algorithm may converge within the destructive combining area resulting in considerably lower system performance.